1. Field of the Invention
The present invention relates to GPS (Global Positioning System) receivers receiving transmission signals from satellites and demodulating data included in the transmission signals. The present invention particularly relates to a GPS receivers receiving a transmission signal from a satellite after acquisition of the satellite while tracking the satellite and demodulating data included in a reception signal every predetermined cycle.
2. Description of the Background Art
The GPS receiver receives a transmission signal from a satellite and demodulates data included in the transmission signal every predetermined cycle. Processes performed by such GPS receiver mainly include an acquisition process for acquiring a desired satellite among a plurality of satellites and a tracking process for demodulating the transmission signal transmitted from the satellite every predetermined cycle after the acquisition of the desired satellite while tracking the satellite.
Generally, in the satellite acquisition process, the GPS receiver calculates a correlation value of a reception signal and a pseudo noise signal (hereinafter referred to as PN signal) corresponding to a number of the satellite, while changing a code phase and a carrier frequency. The GPS receiver determines whether the satellite has been acquired or not based on an amount of energy of the correlation value. In the satellite tracking process after the acquisition of the desired satellite in the above described manner, the GPS receiver generally calculates a correlation value every predetermined cycle (hereinafter referred to as a correlation value output interval) in the same manner as in the acquisition process, and performs a data demodulating process based on the magnitude and the sign of the correlation value.
With reference to FIG. 9, a GPS receiver performing such tracking process (hereinafter referred to as tracking-type GPS receiver) includes a receiving device 101 receiving a transmission signal transmitted from a satellite, a carrier signal generation device 102 generating a carrier signal, a multiplier 103 connected to receiving device 101 and carrier signal generation device 102, having one input receiving a reception signal and another input receiving the carrier signal, a pseudo noise signal generation device 104 generating a PN signal, a correlation value calculation device 105 connected to multiplier 103 and pseudo noise signal generation device 104, calculating a correlation value of an output of multiplier 103 and the PN signal, and a data demodulation device 106 connected to an output of correlation value calculation device 105 performing a data demodulation process based on the magnitude and the sign of the correlation value.
With reference to FIG. 10, the relation of a period spent for correlation value calculation and timing of the correlation value output in the tracking-type GPS receiver shown in FIG. 9 will be described. As can be seen from the drawing, the transmission signal from the satellite is demodulated every predetermined cycle (correlation value output interval) in the tracking process. Therefore, the correlation value is set to be output every correlation value output interval.
In addition, even if the period spent for correlation value calculation is set shorter than the correlation value output interval mentioned above in the tracking process, the tracking-type GPS receiver could not perform data demodulation before the next cycle. Therefore, as far as the calculation of the correlation value is performed during the correlation value output interval, there would be no problem. In other words, even if the correlation value were calculated in a shorter period than the correlation value output interval, the process speed would not be improved. On the other hand, it is well known that when the correlation value calculation period (time spent for calculation of the correlation value by multiplication of PN signal) is shortened, receiving sensitivity of the signal degrades accordingly, and conversely, when the correlation value calculation period is lengthened, the receiving sensitivity of the signal is improved accordingly. Therefore, the probability of successful data demodulation is increased as the correlation value calculation period lengthens.
When the correlation value calculation period is set equal to the correlation value output interval during which demodulation is performed at the tracking process, a correct and fast data demodulation will be realized at the tracking. Hence, conventionally the correlation value calculation period is set equal to the correlation value output interval.
Japanese Patent Laying-Open No. 7-140224 discloses a technique for realizing a high-speed acquisition process by changing a time for multiplication of a PN signal according to a reception signal received by a GPS receiver and reducing a time for correlation value calculation. In the acquisition process, decision of acquisition is executed in accordance with the correlation value. Therefore, a fast acquisition can be realized by shortening the correlation value calculation period. In the tracking process, however, dissimilar to the acquisition process, the fast process cannot be achieved in the same manner because the data demodulation is performed every predetermined cycle.
The minimum correlation value calculation period necessary for data demodulation is not always fixed and dynamically changes according to the condition of the reception signal. When the condition of the reception signal is good, for example, the transmission signal can be demodulated with a short correlation value calculation period.
Hence, when the correlation value calculation period is set equal to the correlation value output interval as in the conventional tracking-type GPS receiver, a longer time period than a required minimum correlation value calculation period is spent for the calculation of correlation value when the condition of the reception signal is good. This means that a correlation value calculation device and so on operate for an extra time. As a result, an extra power is consumed by the operation of this period and the extra power consumption continues until the tracking process completes.